In photometry, luminous intensity is a measure of the
wavelength-weighted power emitted by a light source in a particular
direction per unit solid angle, based on the luminosity function, a
standardized model of the sensitivity of the human eye. The
SI unitSI unit of
luminous intensity is the candela (cd), an SI base unit.
Photometry deals with the measurement of visible light as perceived by
human eyes. The human eye can only see light in the visible spectrum
and has different sensitivities to light of different wavelengths
within the spectrum. When adapted for bright conditions (photopic
vision), the eye is most sensitive to greenish-yellow light at
555 nm.
LightLight with the same radiant intensity at other
wavelengths has a lower luminous intensity. The curve which measures
the response of the human eye to light is a defined standard, known as
the luminosity function. This curve, denoted V(λ) or

y
¯

(
λ
)

displaystyle textstyle overline y (lambda )

, is based on an average of widely differing experimental data from
scientists using different measurement techniques. For instance, the
measured responses of the eye to violet light varied by a factor of
ten[citation needed] .

Relationship to other measures[edit]
Luminous intensityLuminous intensity should not be confused with another photometric
unit, luminous flux, which is the total perceived power emitted in all
directions.
Luminous intensityLuminous intensity is the perceived power per unit solid
angle. If a lamp has a 1 lumen bulb and the optics of the lamp are set
up to focus the light evenly into a 1 steradian beam, then the beam
would have a luminous intensity of 1 candela. If the optics were
changed to concentrate the beam into 1/2 steradian then the source
would have a luminous intensity of 2 candela. The resulting beam is
narrower and brighter, though its luminous flux remains unchanged.
Luminous intensityLuminous intensity is also not the same as the radiant intensity, the
corresponding objective physical quantity used in the measurement
science of radiometry.
Units[edit]
Like other SI base units, the candela has an operational
definition—it is defined by the description of a physical process
that will produce one candela of luminous intensity. By definition, if
one constructs a light source that emits monochromatic green light
with a frequency of 540 THz, and that has a radiant intensity of
1/683 watts per steradian in a given direction, that light source
will emit one candela in the specified direction.[1]
The frequency of light used in the definition corresponds to a
wavelength of 555 nm, which is near the peak of the eye's
response to light. If the source emitted uniformly in all directions,
the total radiant flux would be about 18.40 mW, since there are
4π steradians in a sphere. A typical candle produces very
roughly one candela of luminous intensity.
Prior to the definition of the candela, a variety of units for
luminous intensity were used in various countries. These were
typically based on the brightness of the flame from a "standard
candle" of defined composition, or the brightness of an incandescent
filament of specific design. One of the best-known of these standards
was the English standard: candlepower. One candlepower was the light
produced by a pure spermaceti candle weighing one sixth of a pound and
burning at a rate of 120 grains per hour. Germany, Austria, and
Scandinavia used the Hefnerkerze, a unit based on the output of a
Hefner lamp.[2] In 1881,
Jules ViolleJules Violle proposed the
ViolleViolle as a unit of
luminous intensity, and it was notable as the first unit of light
intensity that did not depend on the properties of a particular lamp.
All of these units were superseded by the definition of the candela.
Usage[edit]
The luminous intensity for monochromatic light of a particular
wavelength λ is given by

^ Standards organizations recommend that photometric quantities be
denoted with a suffix "v" (for "visual") to avoid confusion with
radiometric or photon quantities. For example: USA Standard Letter
Symbols for Illuminating Engineering USAS Z7.1-1967, Y10.18-1967
^ The symbols in this column denote dimensions; "L", "T" and "J" are
for length, time and luminous intensity respectively, not the symbols
for the units litre, tesla and joule.
^ a b c Alternative symbols sometimes seen: W for luminous energy, P
or F for luminous flux, and ρ or K for luminous efficacy.